Compose key magic

There are some amusing things in Linux which are hard to discover because they are invisible. Entering Unicode characters and sequences with a Compose key is one of those hidden features which can make user’s life much easier.

How it works

It’s simple and easy. You press and release a special key called “Compose” (or, “Multi_key”) and then, two or three character keys – a mnemonic ASCII sequence identifies a Unicode character you want to see. For example, the sequence of letter keys “o” and “c” produces the copyright symbol (©) and it is much easier to remember than Unicode 00A9.

Where is my Compose key?

There is no Compose key defined in X.org by default. Before using it, you need to do some configuration to tell X which key you want to use as a Compose. Usually, right “Win” (“Super”) key is a good choice.

In KDE, open “Keyboard layout” configuration panel (Control Center → Regional & Accessibility → Keyboard Layout) and select “Enable xkb options” checkbox on “Xkb options” tab. Then scroll the options list down to “Compose key position” section and select a checkbox for a key you want to use as a Compose.

Alternatively, you can edit /etc/X11/Xorg.conf (as a root) to add the line to the keyboard section:

Option "XkbOptions"  "compose:rwin"

After configuration is done, restart X (simply hit Ctrl+Alt+Backspace).

Note that you’ll need Unicode fonts installed (to see all exotic characters) and a default locale with UTF-8 support (e.g. en_US.UTF-8 for US English). This seems to be the default in modern Linux distributions.

Predefined characters

Diacritics, ligatures and currency symbols

Entering extended latin letters with diacritic marks follows the pattern:

Compose diacritic_character letter

The diacritic characters are:

• ' (apostrophe) ⇒ A letter with acute (Áá)
• " (double quote) ⇒ A letter with diaeresis (Ää)
• ` (grave) ⇒ A letter with grave (Àà)
• _ (underscore) ⇒ A letter with macron (Āā)
• ^ (circumflex) ⇒ A letter with circumflex (Ââ)
• ~ (tilde) ⇒ A letter with tilde (Ãã)
• , (comma) ⇒ A letter with cedilla (Çç)
• . (period) ⇒ A letter with dot above (Ȧȧ)
• ! (exclamation) ⇒ A letter with dot below (Ạạ)
• ? (question) ⇒ A letter with hook (Ảả)
• / (slash), - (minus) ⇒ A letter with stroke (Øø, Đđ)
• ; (semicolon) ⇒ A letter with ogonek (Ąą)
• + (plus) ⇒ A letter with horn (Ơơ)
• = (equals) ⇒ A letter with double acute (Őő)
• o ⇒ A letter with ring (Åå)
• c ⇒ A letter with caron (Ǎǎ)
• b ⇒ A letter with breve (Ăă)

Some characters may be combined to create mixed diacritical marks, e.g. Compose _ " a produces ǟ (‘a’ with diaeresis and macron), and so on.

Other letters and ligatures:

• Compose A/a E/e ⇒ Æ/æ
• Compose O/o E/e ⇒ Œ/œ
• Compose T/t H/h ⇒ Þ/þ
• Compose N/n G/g ⇒ Ŋ/ŋ
• Compose s s ⇒ ß
• Compose e e ⇒ ə

Currency symbols:

• Compose c / ⇒ ¢
• Compose C = ⇒ €
• Compose L = ⇒ ₤
• Compose F r ⇒ ₣
• Compose Y = ⇒ ¥
• Compose o x ⇒ ¤

Punctuation marks and other symbols

Quotation marks:

• Compose , ' ⇒ ‚
• Compose , " ⇒ „
• Compose < ' ⇒ ‘
• Compose > ' ⇒ ’
• Compose < " ⇒ “
• Compose > " ⇒ ”
• Compose < < ⇒ «
• Compose > > ⇒ »
• Compose . < ⇒ ‹
• Compose . > ⇒ ›

Others:

• Compose . . ⇒ ·
• Compose - - - ⇒ — (em-dash)
• Compose - - . ⇒ – (en-dash)
• Compose ? ? ⇒ ¿
• Compose ! ! ⇒ ¡
• Compose + - ⇒ ±
• Compose : - ⇒ ÷
• Compose 1 2 ⇒ ½
• Compose 1 4 ⇒ ¼
• Compose 3 4 ⇒ ¾
• Compose _ 0-9 ⇒ ₀ – ₉ (subscript digit)
• Compose ^ 0-9 ⇒ ⁰ – ⁹ (superscript digit)
• Compose ( 0-9 ) ⇒ ⓪ – ⑨ (circled digit)
• Compose o c ⇒ ©
• Compose o r ⇒ ®
• Compose o o ⇒ °
• Compose o s ⇒ §
• Compose x x ⇒ ×
• Compose P P ⇒ ¶
• Compose T M ⇒ ™
• Compose m u ⇒ µ
• Compose % o ⇒ ‰

Defining custom Compose sequences

Default Compose sequences are defined in ‘Compose‘ text file in the current locale directory (/usr/share/X11/locale/xxx). This file contains the rules to define Compose keyboard sequences and corresponding Unicode characters:

# UTF-8 (Unicode) compose sequence
# David.Monniaux (at) ens.fr
#
# $XFree86: xc/nls/Compose/en_US.UTF-8,v 1.11 2004/01/06 13:14:04 pascal Exp $
...
<Multi_key> <less> <less>        	: "«"   guillemotleft # LEFT-POINTING DOUBLE ANGLE QUOTATION MARK
<Multi_key> <greater> <greater>  	: "»"   guillemotright # RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK
<Multi_key> <less> <apostrophe>  	: "‘"   U2018 # LEFT SINGLE QUOTATION MARK
...

To define new Compose sequence, we should add another rule using the syntax:

<Multi_key> <key1> [<key2> ...]   	: "character"

For instance, for a smiley character (☺, Unicode 263A), the rule could be

<Multi_key> <colon> <parenright> : "☺"  # Compose : )

There is a number of utilities (like KCharSelect in KDE) for to select the Unicode characters and insert them into the file via clipboard.

It is possible to add new rules just into this system file (if you are root), but it’s better to create ‘.XCompose‘ file in the user home directory:

# ~/.XCompose
# This file defines custom Compose sequence for Unicode characters 

# Import default rules from the system Compose file:
include "/usr/share/X11/locale/en_US.UTF-8/Compose"

<Multi_key> <colon> <parenright> : "☺" U263A   # Compose : )
<Multi_key> <minus> <less>   : "←"  U2190 # Compose - <
<Multi_key> <minus> <greater> : "→" U2192 # Compose ->
...

You also can assign whole strings to the Compose sequences — for instance, syntax constructions of your favorite programming language or HTML tags:

<Multi_key> <less> <p> : "<p></p>" # Compose < p
<Multi_key> <less> <a> : "<a href=""></a>" # Compose < a
...